1. Field of the Invention
The present invention relates to a liquid crystal display and, more particularly, to a reflective or semi-transmissive liquid crystal display having alignment regulating structures for regulating the alignment of the liquid crystal.
2. Description of the Related Art
Recently, active matrix liquid crystal displays having a thin film transistor (TFT) at each pixel are widely used as displays in all kinds of applications. Under such a circumstance, semi-transmissive (transflective) liquid crystal displays capable of displaying in both of reflective and transmissive modes have come in use as displays for mobile terminals or notebook PCs.
FIG. 7 shows a configuration of one pixel of a semi-transmissive liquid crystal display panel utilizing a technique proposed in a Japanese patent application (Application No. 2003-410742) made by the applicant. FIG. 8A is a view of a sectional configuration of the liquid crystal display panel taken along the line X-X in FIG. 7, and FIG. 8B is a view of a sectional configuration of the liquid crystal display panel taken along the line Y-Y in FIG. 7. As shown in FIGS. 7 to 8B, the liquid crystal display panel has a TFT substrate 102, an opposite substrate 104, and a liquid crystal layer 106 sealed between the substrates 102 and 104. The TFT substrate 102 includes a plurality of gate bus lines 112 formed on a glass substrate 110 and a plurality of drain bus lines 114 extending across the gate bus lines 112 with an insulation film 130 interposed between them. Channel protection film type TFTs 120 are formed in the vicinity of intersections between the gate bus lines 112 and the drain bus lines 114. A protective film 131 is formed on the TFT 120. Storage capacitor bus lines 118 are formed so as to extend across pixel regions surrounded by the gate bus lines 112 and the drain bus lines 114 and in parallel with the gate bus lines 112.
Each of the pixel regions is generally divided into three areas, and the region thus includes a reflective area disposed in the middle and two transmissive areas disposed above and below the reflective area in the view in FIG. 7. A reflective plate 150 is formed above the insulation film 130 in the reflective area, the reflecting plate being formed from the same material and in the same layer as the drain bus lines 114. A plurality of openings 118a is formed in a region of the storage capacitor bus line 118 overlapping the reflective plate 150. On both sides of the storage capacitor bus line 118, there are two metal layers 152 which are formed from the same material and in the same layer as the storage capacitor bus line 118 and each of which is electrically isolated from the storage capacitor bus line 118. A plurality of openings 152a is formed in the metal layers 152. A plurality of dielectric layers 153 is provided on the metal layers 152, the dielectric layers being formed from the same material and in the same layer as the channel protection film of the TFT 120. Since the storage capacitor bus line 118 formed with the openings 118a, the metal layers 152 formed with the openings 152a, and the dielectric layer 153 are formed under the reflective plate 150, irregularities in a shape following those elements are formed on the surface of the reflective plate 150.
Transparent pixel electrodes 116 having a predetermined shape are formed in the pixel regions above the protective film 131. The pixel electrodes 116 in the reflective area and the transmissive areas of one pixel are electrically connected to each other. The pixel electrodes 116 are electrically connected to a source electrode of the TFT 120 through a contact hole 125. The pixel electrodes 116 are electrically connected to the reflective plate 150 through an opening 132 providing by etching the protective film 131 on the reflective plate 150.
The opposite substrate 104 includes a common electrode 141 formed on a glass substrate 111. Point-like protrusive structures 142 formed on the common electrode 141 substantially in the middle of the transmissive areas as alignment regulating structures for regulating the alignment of liquid crystal molecules of the liquid crystal layer 106. A columnar spacer 143 for maintaining a cell gap is formed on the common electrode 141 substantially in the middle of the reflective area. The columnar spacer 143 also serves as an alignment regulating structure. A transparent resin layer may be formed between the glass substrate 111 and the common electrode 141 in the reflective area to reduce the cell gap in the reflective area to substantially one-half of the cell gap in the transmissive areas. The liquid crystal display panel shown in FIGS. 7 to 8B can achieve relatively high display quality in both of reflective and trasmissive modes.
However, the liquid crystal display panel shown in FIGS. 7 to 8B has a problem in that variation of a cell thickness (variation attributable to gravity) occurs when the panel is erected such that substrate surfaces are substantially in parallel with the vertical direction and left under the high temperature, the variation being a greater cell thickness in a lower part of the panel in comparison to that in an upper part. It is assumed that variation attributable to gravity occurs because the columnar spacer 143 is formed at all pixels. Specifically, a substrate combining step for combining the TFT substrate 102 and the opposite substrate 104 is performed with a predetermined pressure applied between the substrates 102 and 104. When the columnar spacers 143 are formed at all pixels, no high pressure is applied to each columnar spacer 143. Therefore, substantially no compression occurs at each columnar spacer 143. As a result, when the cell thickness increases as a result of thermal expansion of the volume of the liquid crystal sealed between the substrates 102 and 104, the columnar spacers 143 cannot deform in compliance with the same. When the height of the columnar spacers 143 is smaller than the increased cell thickness, the columnar spacers 143 leave either of the substrates. As a result, when the liquid crystal display panel is erected, the liquid crystal in an upper part of the panel moves down to a lower part of the panel due to gravity, and the cell thickness thus becomes greater in the lower part than in the upper part.
A possible way to simply suppress an irregularity of a cell thickness is to reduce the density in which the columnar spacers 143 are disposed. However, in the configurations shown in FIG. 7 to FIG. 8B, since the columnar spacers 143 also serve as an alignment regulating structure, it is difficult to reduce the density in which the columnar spacer 143 are disposed.
Patent Document 1: JP-A-2000-267121
Patent Document 2: JP-A-2003-241185